The present invention relates to a process for the preparation of a 2-position substituted-3,3-difluorofuran using diethylammonium sulfur trifluoride (DAST) reacted with a 2-position substituted furan-3-one. In particular the present invention relates to the preparation of a 2',2'-difluoronucleoside, particularly where the nucleobase is a pyrimidine such as cytosine.
A particular compound which is prepared by the process is 1-(2',2'-difluoro-.beta.-D-arabinofuranosyl)cytosine (2',2'-difluorocytidine) from cytidine. This compound is an anticancer and antiviral agent.
A strategy for the synthesis of 1-(2',2'-difluoro-.beta.-D-arabinofuranosyl)cytosine (2',2'-difluorocytidine) is the introduction of the 2',2'-difluoro groups into a suitably protected nucleoside precursor as shown in the following reaction Scheme 1. ##STR1##
An economic analysis based on Scheme 1 shows that cytidine is a relatively inexpensive, commercially available starting material and thus Scheme 1 can be economically beneficial.
There are two primary issues associated with Scheme 1. The first of these is the development of an efficient protection scheme for cytidine that differentiates the 2'- and 3'-hydroxyl groups, and the provision of protecting groups (Pg) which survive a fluorination reaction. The second, more difficult problem is whether a suitably mild and selective process can be found for effecting geminal difluorination at C-2' of a 2'-ketonucleoside.
The peracylation, selective 2'-deacylation strategy reported by Nishino, et al., Tetrahedron, 42(7), 1995-2004 (1986) was successfully employed to address the protection-differentiation issue. The conversion of cytidine to the corresponding 2'-keto-3',5',N.sup.4 -protected compound 3 was accomplished in three steps with an overall yield of 53% as outlined in reaction Scheme 2. ##STR2## a. o-tol-Cl (5 eq), pyridine. b. KOt-Bu (4 eq), THF, -78.degree. C. c. PDC (0.7 eq), Ac.sub.2 O (3 eq), CH.sub.2 Cl.sub.2, 25.degree. C. regiochemical purity by direct crystallization. It was found that the reported conditions (5 eq KOt-Bu, CH.sub.2 Cl.sub.2, -20.degree. C.) (Nishino, et al., Tetrahedron, 42, 1995-2004 (1986)) for the selective deacylation of compound 1 were unsatisfactory. However, under modified conditions (4 eq KOt-Bu, THF, -78.degree. C., 1 hour) an 84:16 mixture of 2'- and 3'-deacylated nucleosides was obtained, respectively, from which the desired product, compound 2, was isolated in 74% yield and &gt;98% purity.
Oxidation of 2 with PDC/Ac.sub.2 O (Andersson, et al., Carbohydrate Res,, 129, C1-C3 (1984)) gave the desired compound 3 in 82% yield.
The most convenient and direct method for the conversion of a carbonyl function into a gem difluoride is by treatment with one of the family of dialkylamino sulfur trifluoride reagents, of which the diethyl analog (DAST) is the most widely used example (Hudlicky, in "Organic Reactions", Wiley & Sons: New York, Vol. 35, Chapter 3, p. 513 (1988)). The reaction is shown in Scheme 3. ##STR3##
When compound 3 was treated with excess DAST in benzene at 25.degree. C. for 70 hours most of the starting compound 3 was recovered intact. Under more forcing conditions, i.e. 80.degree. C./16 hours or 110.degree. C./16 hours, extensive degradation of compound 3 was observed. In none of these experiments was a significant amount (&lt;0.5%) of the difluoride compound 4 detected. The reaction mixtures were assayed by reverse-phase HPLC after an aqueous workup.
In a related experiment, the 2'-hydroxy nucleoside 2 was treated with excess dimethylaminosulfur trifluoride at 25.degree. C. (Scheme 4). ##STR4##
The only product that was identified in the resulting mixture was the corresponding ara-cytidine compound 5, which was isolated in 27% yield. The formation of this C-2'-inverted alcohol, instead of the corresponding 2',2'-difluoride, under these conditions strongly implicates the intermediacy of an O.sup.2,2'-cyclonucleoside, which hydrolyzed to the derivative 5 on workup, as depicted in Scheme 4. The formation of such pyrimidine cyclonucleosides is well precedented, See: Goodman, L., in Basic Principles in Nucleic Acid Chemistry, Vol. 1; Ts'o, P. O. P., Ed.; Academic: New York, Chapter 2, pp 177-190 and references therein (1974)).
A related cyclonucleoside intermediate which could form on treatment of the 2'-keto compound 3 in Scheme 3 with DAST would regenerate the starting ketone during aqueous workup. After confirming the unreactivity of compound 3 toward DAST, and weakening the steric argument by the use of compound 3b shown in Scheme 5, the cause of the failure of the reaction was uncertain. One potential source of a failure to isolate products would be occurrence of the reaction shown in Scheme 5. DAST treatment results in formation of an internal geminal fluorohydrin ether. This molecule would decompose to starting material on aqueous workup, therefore no net reaction would be observed. ##STR5##
In the experiments, no evidence of the reaction in Scheme 5 was collected. The treatment of 3b with DAST was done in an NMR tube and monitored in situ. Formation of an intermediate which is stable until workup should have been observable. In fact, no shifts were observed in the proton NMR signals for molecule 3b which were not obscured by the DAST. Also, no species other than DAST was observed in the .sup.19 F-NMR.
Semi-empirical calculations were used to probe the reactivity differences between model compounds A and B and the 2'-keto compounds 3 and 3b. The difference between compound 3b and compounds A and B is shown by the Mulliken charge density at the oxygen: ##STR6##
Molecule 3b possesses dramatically less charge density at the oxygen of the ketone. If a mechanism is assumed in which the first step of the fluorination is nucleophilic attack of this oxygen, then this lack of charge would explain the lack of reactivity. Further support of this theory was supplied by the difluorination of 3'-keto derivative B in the labs of Bergstrom, et al. (J. Med. Chem., 35, 3369 (1992)). This molecule was calculated to have an electron density at oxygen similar to that of A. ##STR7##
Another method for the conversion of a carbonyl compound to the corresponding 2',2'-difluorocytidine is the reaction of its derived dithioketal with BrF (Sondej, et al., J. Org. Chem., 51, 3508-3513 (1986)). Despite numerous attempts, we were unable to effect the conversion of compound 3 to the requisite ethylene or propylene dithioketal intermediate compound 6 (Scheme 6). ##STR8##
Treatment of compound 3 with ethane or propane dithiol at 25.degree. C. in the presence of BF.sub.3 etherate, Zn(OTf).sub.2, or AlCl.sub.3 afforded no significant reaction. Under more forcing conditions, extensive degradation of compound 3 was observed. In no case was any of the desired product detected. Under two sets of conditions, ring cleavage products were isolated and identified (Schemes 7 and 8). ##STR9##
The formation of compound 7 under these conditions pointed to a serious selectivity problem and the process was abandoned.
There was a need for a fluorination process to prepare 2',2'-difluorocytidine from the corresponding 2'-ketocytidine. This process has potential for a much shorter and economical synthesis of 2-substituted-3,3-difluorofuran. The problem was to provide a process for performing the fluorination.
It is therefore an object of the present invention to provide a process for the fluorination of the 2-position substituted furan-3-one to produce a 2-position substituted-3,3-difluorofuran using DAST. The present invention particularly relates to a process for preparing 1-(2',2'-difluoro-3',5'-di-O-acetyl-.beta.-D-arabinofuranosyl)-N.sup.4 -acetylcytosine (2',2'-difluorocytidine). Further, it is an object of the present invention to provide a process which provides the 2',2'-difluorocytidine in high yield. These and other objects will become increasingly apparent by reference to the following description.